Method of roll-forming high-strength aluminum alloy and roll-formed product using the same

ABSTRACT

A method of roll-forming a high-strength aluminum alloy, may include processing an aluminum alloy plate to form a square bar, pressing the plate so that deformation begins in a cross section thereof; and forming the square bar having at least one bent portion by completion of the deformation, wherein the method further includes partially performing a heat treatment on the bent portion before forming the square bar.

CROSS-REFERENCE(S) TO RELATED APPLICATIONS

The present application claims priority to Korean Patent Application No.10-2016-0173538, filed on Dec. 19, 2016, the entire contents of which isincorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION Field of the Invention

Exemplary The present invention relates to a method of roll-forming ahigh-strength aluminum alloy and a roll-formed product using the same;and, more particularly, to a method of locally softening androll-forming a high-strength aluminum alloy, configured to enhance aformability of the alloy by performing a local heat treatment process ona bent portion, on which stress is concentrated, during the formingprocess and improves a resistance against stress corrosion cracking byremoving internal residual stress from the bent portion, and aroll-formed product using the same.

Description of Related Art

An aluminum alloy is conventionally expressed as a 4-digit number from1000 to 8000 series according to an alloying element thereof.

A strength of the aluminum alloy is determined by the alloying elements.The alloying elements including copper (Cu), manganese (Mn), zinc (Zn),and silicon (Si) have increasing solubility as a temperature of thealuminum alloy increases. Accordingly, the alloying elements may beprecipitated and hardened by a heat treatment and a solution treatment.In the present case, the aluminum alloy with the alloying elements addedthereto is referred to as a heat-treated aluminum alloy.

A 7000 series aluminum alloy is an Al—Zn—(Mg and/or Cu) alloy, and is ahigh-strength heat-treated alloy that includes Zn as a main componentand Mg added thereto. In particular, Al 7075 alloy has the higheststrength, namely a tensile strength of 550 MPa, from among the aluminummaterials.

However, there is a problem in that the aluminum alloys lack a desiredformability due to having a high strength during a cold forming processas well as having a very low resistance to a stress corrosion crackingdue to residual stress.

Due to the stress corrosion cracking, aircraft products have beenmanufactured by cutting bulk materials instead of forming processes,including pressing, even when the 7000 series aluminum alloy is applied.

The information disclosed in this Background of the Invention section isonly for enhancement of understanding of the general background of theinvention and should not be taken as an acknowledgement or any form ofsuggestion that this information forms the prior art already known to aperson skilled in the art.

BRIEF SUMMARY

Various exemplary embodiments of the present invention are directed toproviding a method of locally softening and roll-forming a high-strengthaluminum alloy, configured to enhance a formability of the alloy byperforming a local heat treatment process on a bent portion, on whichstress is concentrated, during the forming process as well as improves aresistance to stress corrosion cracking by removing residual stress inthe bent portion, and a roll-formed product using the same.

Other objects and advantages of the present invention can be understoodby the following description, and become apparent with reference to theexemplary embodiments of the present invention. Also, it is obvious tothose skilled in the art to which the present invention pertains thatthe objects and advantages of the present invention can be realized bythe means as claimed and combinations thereof.

In accordance with various exemplary embodiments of the presentinvention, a method of roll-forming high-strength aluminum alloyincludes processing an aluminum alloy plate to form a square bar,pressing the plate wherein deformation begins in a cross sectionthereof, and forming the square bar having at least one bent portion bycompletion of the deformation process, wherein the method may furtherinclude partially performing a heat treatment on the bent portion beforethe forming of the square bar.

The heat treatment may be performed by irradiating the bent portion witha laser for a predetermined time period.

The heat treatment may be performed by heating the bent portion at atemperature of 350 to 400° C.

The aluminum alloy may be a 7000 series aluminum alloy.

In accordance with various exemplary embodiments of the presentinvention, a roll-formed product of a high-strength aluminum alloy, theroll-formed product including an aluminum alloy and having a bentportion formed thereon, the roll-formed product is formed by partiallyperforming a heat treatment on the bent portion to remove a residualstress which is partially generated in the bent portion during theforming process.

The heat treatment may be performed by irradiating the bent portion withthe laser for a predetermined time period.

The heat treatment may be performed by heating the bent portion at atemperature of 350 to 400° C.

The aluminum alloy may be a 7000 series aluminum alloy.

The product may be configured wherein a microstructure thereof includesa ratio of a low angle boundary with a tilt angle equal to or greaterthan 2° and less than 15° is equal to or less than 21%.

The product may be a bumper beam for a vehicle.

The methods and apparatuses of the present invention have other featuresand advantages which will be apparent from or are set forth in moredetail in the accompanying drawings, which are incorporated herein, andthe following Detailed Description, which together serve to explaincertain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically illustrating a method of roll-forming ahigh-strength aluminum alloy according to an exemplary embodiment of thepresent invention;

FIG. 2A is a view schematically illustrating a roll-formed product of ahigh-strength aluminum alloy and a bent portion of the product accordingto an exemplary embodiment of the present invention;

FIG. 2B is a cross-sectional view taken along line A-A′ of FIG. 2A;

FIG. 3 is a graph illustrating a change of true strain according to aheat treatment temperature in a method of roll-forming a high-strengthaluminum alloy according to an exemplary embodiment of the presentinvention;

FIG. 4 is a graph illustrating a change of strength according to a heattreatment temperature in a method of roll-forming a high-strengthaluminum alloy according to an exemplary embodiment of the presentinvention;

FIG. 5 is a graph illustrating a change of low angle boundary ratioaccording to a heat treatment temperature in a method of roll-forming ahigh-strength aluminum alloy according to an exemplary embodiment of thepresent invention;

FIG. 6 illustrates a result of stress corrosion cracking according to aheat treatment temperature in a method of roll-forming a high-strengthaluminum alloy according to an exemplary embodiment of the presentinvention; and

FIG. 7 illustrates an incidence of cracks on cross-sectional portionsaccording to a heat treatment temperature in a roll-formed product of ahigh-strength aluminum alloy and a bent portion of the product accordingto an exemplary embodiment of the present invention.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variousfeatures illustrative of the basic principles of the invention. Thespecific design features of the present invention as disclosed herein,including, for example, specific dimensions, orientations, locations,and shapes will be determined in part by the particular intendedapplication and use environment.

In the figures, reference numbers refer to the same or equivalent partsof the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of thepresent invention(s), examples of which are illustrated in theaccompanying drawings and described below. While the invention(s) willbe described in conjunction with exemplary embodiments, it will beunderstood that the present description is not intended to limit theinvention(s) to those exemplary embodiments. On the contrary, theinvention(s) is/are intended to cover not only the exemplaryembodiments, but also various alternatives, modifications, equivalentsand other embodiments, which may be included within the spirit and scopeof the invention as defined by the appended claims.

Prior to description of exemplary embodiments of the present invention,various aspects of the present invention are directed to providing astress removal and formability improvement with respect to a bentportion during roll-forming of a 7000 series aluminum alloy plate. Ingeneral, after a solution heat treatment is performed on the 7000 seriesaluminum alloy at a temperature of approximately 490 to 530° C. for asolution of alloying elements including Cu, Mg, and Si, the 7000 seriesaluminum alloy is rapidly cooled to room temperature, obtaining asupersaturated solid solution. As such, a second phase is precipitatedby performing an artificial or a natural aging treatment on thesupersaturated solid solution, wherein the 7000 series aluminum alloyhas a maximum strength.

However, the 7000 series aluminum alloy lacks formability due to havinga high strength and is prone to stress corrosion cracking (SCC) due tointernal stress generated in portions of the ally in a forming process.The stress corrosion cracking is a phenomenon in which cracks occur andspread when a tensile stress is applied to a specific material in aspecific corrosion environment, and occurs when conditions of corrosionenvironment, sensitive alloy, and stress are simultaneously satisfied.In particular, it is known that the stress corrosion cracking occurswhen the element Cu of a high-strength 7000 series aluminum alloy(Al—Zn—Mg—Cu) forms an intermetallic compound, including MgCu₂ orAlMgCu₂, and pitting corrosion is induced in the corrosion environment.

The present invention is directed to prevent stress corrosion crackingby mitigating the stress condition from among the above threeconditions, and the method thereof will be described later.

FIG. 1 is a view schematically illustrating a method of roll-forming ahigh-strength aluminum alloy according to an exemplary embodiment of thepresent invention.

Referring to FIG. 1, the method of roll-forming the high-strengthaluminum alloy according to the exemplary embodiment of the presentinvention is a roll-forming method of processing an aluminum alloy plate200 to form a square bar, and includes a step of pressing the plate 200wherein deformation begins in a cross section thereof, and a step offorming the square bar having at least one bent portion 410 bycompletion of the deformation process. In the present case, the methodmay further include a step of partially performing a heat treatment 210only at the bent portion 410, which is formed by bending the plate 200,before the square bar is formed having a predetermined shape and afterthe plate 200 is bent, wherein the deformation begins in the crosssection thereof.

The method of roll-forming the aluminum alloy is continuously performedwherein the rolled plate 200 has a constant thickness and passes betweenupper and lower rollers 310 and 320 to have the predetermined shape.However, when the plate 200 is bent at a time at a desired angle, theplate may be cracked due to the strength thereof. Therefore, after thedeformation of the plate 200 begins, the method allows the plate to besequentially deformed through a plurality of shape rolling rollers 300,finally forming a desired shaped square bar.

In a case where a square bar having a plurality of bent portions 410 isformed by bending a 7000 series aluminum alloy plate, due to thestrength thereof and as in an exemplary embodiment of the presentinvention, the 7000 series aluminum alloy lacks a desired formability ina forming process and the stress corrosion cracking also occurs in afinal molded product due to stress concentrated on the bent portions.

Accordingly, the method according to the exemplary embodiment of thepresent invention improves the formability while curing dislocationscaused during the forming by partially heating 210 only the bent portion410 on which stress is concentrated in the forming process, and preventscracks from occurring in the stress concentration portion in thecorrosion environment by removing internal stress therefrom.

The provided heat treatment 210 step should be performed within a shorttime due to the roll-forming process which is continuously performed,and may be performed through accurate control of only a desired portionat an accurate temperature. The bent portion 410 is preferablyirradiated with a laser 100 for a certain time in the exemplaryembodiment of the present invention. In the instant case, since thelaser which is usable in conjunction with a 6-axis robot is configuredto carry out rapid heating and has a variety of beam sizes, it isconfigured to freely adjust a range of heat treatment.

When the local heat treatment 210 is performed on the bent portion 410at too high a temperature, the solution heat treatment as a mechanismfor reinforcing the 7000 series aluminum alloy is released, which maylead to a low strength, as described above. On the other hand, when thelocal heat treatment is performed at too low a temperature, the removaleffect of residual stress may be very slight. Accordingly, the heattreatment step is preferably performed wherein the bent portion isheated at a temperature of 350 to 400° C. in the exemplary embodiment ofthe present invention.

Typically, a material may be irradiated with the laser 100 forapproximately 2 seconds, according to a process speed of roll forming,and is then cooled slowly in the continuous roll forming process.

FIG. 2 is a view schematically illustrating a roll-formed product of ahigh-strength aluminum alloy 400 and a bent portion 410 of the product400 according to an exemplary embodiment of the present invention.

Referring to FIG. 2, the roll-formed product of high-strength aluminumalloy 400 according to the exemplary embodiment of the present inventionis a roll-formed product that includes an aluminum alloy and has thebent portion 410 formed thereon. The core technique of the roll-formedproduct of high-strength aluminum alloy 400 according to the exemplaryembodiment of the present invention the present invention performspartial heat treatment on the bent portion 410 to remove residual stresswhich is concentrated on the bent portion 410 during the forming.

As described above, the heat treatment is performed by irradiating thebent portion 410 with the laser 100 for a certain time and heating thebent portion 410 at a temperature of 350 to 400° C. The bent portion 410may be typically irradiated with laser for about 2 seconds according tothe process speed of roll forming, but the irradiation time isadjustable according to the process speed. For example, the irradiationtime is not limited as long as the temperature is suitably adjusted.

In addition, the product 400 is characterized wherein the microstructurethereof, the boundary where a difference in orientation of adjacentcrystal grains is equal to or greater than 2° and less than 15° isdefined as a low angle boundary, and a ratio of the low angle boundaryis equal to or less than 21%.

The greater the ratio of the low angle boundary, the smaller a diameterof each subgrain generated in the particles. As seen in the followingEquation 1, the smaller the diameter of the subgrain, the greater theresidual stress in a material. Accordingly, the ratio of the low angleboundary is preferably equal to or less than 21% as illustrated inTable 1. In the present case, it can be seen that the residual stress isadequately removed from the bent portion 410 to suppress the stresscorrosion cracking.

$\begin{matrix}{{S_{i} = {\alpha \frac{\gamma}{D}}},} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

(where α is a geometric constant, γ is a boundary energy, and D is thediameter of subgrain).

The following Table 1 indicates the stress corrosion cracking,formability, strength, and the low angle boundary ratio according to aheat treatment temperature.

In exemplary embodiments of the present invention and comparativeexamples, crystal grain orientations are measured using an Electron BackScattering Diffraction (EBSD) analyzer. Since crystalline materials havedifferent orientations for each crystal grain, a diffraction ofbackscattered electrons is changed when specimens are irradiated withelectron beams generated by an electron microscope. Each of the crystalgrain orientations is analyzed using kikuchi patterns of thebackscattered electrons. In addition, as described above, the boundarywhere the difference in orientation of adjacent crystal grains is equalto or greater than 2° and less than 15° is defined as the low angleboundary, and the low angle boundary is measured in an exemplaryembodiment of the present invention. It can be seen from Table 1 thatwhen the low angle boundary ratio is equal to or less than 21%, thestress corrosion cracking is suppressed by removal of residual stress.

As seen in Table 1 and FIG. 4, according to a result of an evaluation oftensile strength after the heat treatment is performed on the aluminumplate with the laser for 2 seconds, it can be seen that the strength ismaintained to a temperature of 400° C., but it is significantly loweredat a temperature of 500° C. This is because solution heat treatment fora 7000 series aluminum alloy is released at high temperature and the7000 series aluminum alloy has a low strength.

As seen in Table 1 and FIG. 3, the strain is meaningfully improved at atemperature equal to or greater than 300° C. In FIG. 5, after apre-strain of 5% is applied to an aluminum plate and a heat treatment isperformed on the aluminum plate with the laser for 2 seconds, residualstress is measured using the EBSD analyzer. In the present case, theresidual stress begins to be reduced at a temperature of 250° C. and issignificantly reduced at a temperature of approximately 350° C.

In FIG. 6, whether cracks occur in a bent portion is evaluated by areproduction evaluation method to verify whether stress corrosioncracking occurs according to removal of residual stress. According to aresult of checking whether cracks occur in a bent specimen in acorrosion environment, i.e. in a solution of 3.5% NaCl for 300 hours,the bent specimen is cracked after 1.5 days at a temperature of 250° C.and after 3 days at a temperature of 300° C., yet the bent specimen isnot cracked even after 300 hours at a temperature of 350° C. This meansthat the residual stress is relieved to suppress stress corrosioncracking in the condition in which heat treatment is performed at atemperature equal to or higher than 350° C., and is because the stresscorrosion cracking is prevented by removal of stress from among threefactors that cause the stress corrosion cracking.

In FIG. 7, the stress corrosion cracking is tested on an actual bumperbeam member in the same conditions. In the present case, a crack occursin a bent portion at a temperature of 300° C., but no stress corrosioncracking occurs when the bent portion is formed at a temperature of 350°C. As a result, the present bumper beam member exhibits the same resultsas the above specimen.

TABLE 1 Comp. Comp. Comp. Comp. Present Present Comp. Comp. Sort Example1 Example 2 Example 3 Example 4 Example 1 Example 2 Example 5 Example 6Heat 150 200 250 300 350 400 450 500 Treatment Temperature (° C.) TrueStrain 30 32 33 45 64 82 118 190 (%) Stress 510 506 503 495 489 485 410300 (MPa) Low Angle 47 47 44 39 21 12 9.3 6.3 Boundary Ratio (%)Formability X X X ◯ ◯ ◯ ◯ ◯ SCC X X X X ◯ ◯ ◯ ◯

As described above, in the exemplary embodiment of the presentinvention, the stress concentration portion, i.e. The bent portion islocally heated with the laser at a temperature of 350 to 400° C. in aconventional roll-forming process, with the consequence that it ispossible to improve the resistance against stress corrosion fatigue byremoving residual stress and thus to enhance the formability.

In accordance with exemplary embodiments of the present invention, it ispossible to enhance a formability by performing a local heat treatmenton a bent portion, on which stress is concentrated, during the formingprocess.

In addition, it is possible to improve resistance against stresscorrosion cracking by removing residual stress which is concentrated onthe bent portion.

Furthermore, it is possible to reduce a thicknesses and weight ofproducts to be manufactured, and thus to improve a fuel efficiency byapplication of a high-strength aluminum material thereto.

Moreover, it is possible to improve quality and dimensional accuracy ofproducts by removing the residual stress from the bent portion.

For convenience in explanation and accurate definition in the appendedclaims, the terms “upper”, “lower”, “up”, “down”, “upwards”,“downwards”, “internal”, “outer”, “inside”, “outside”, “inwardly”,“outwardly”, “internal”, “external”, “front”, “rear”, “back”,“forwards”, and “backwards” are used to describe features of theexemplary embodiments with reference to the positions of such featuresas displayed in the figures.

The foregoing descriptions of specific exemplary embodiments of thepresent invention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteachings. The exemplary embodiments were chosen and described toexplain certain principles of the invention and their practicalapplication, to enable others skilled in the art to make and utilizevarious exemplary embodiments of the present invention, as well asvarious alternatives and modifications thereof. It is intended that thescope of the invention be defined in the Claims appended hereto andtheir equivalents.

What is claimed is:
 1. A method of roll-forming an aluminum alloy byprocessing an aluminum alloy plate to form a square bar, the methodcomprising: pressing the plate so that deformation begins in a crosssection thereof; and forming the square bar having at least one bentportion by completion of the deformation, and partially performing aheat treatment on the bent portion before forming the square bar.
 2. Themethod of claim 1, wherein the heat treatment is performed by heatingthe bent portion at a temperature of 350 to 400° C.
 3. The method ofclaim 2, wherein the heat treatment is performed by irradiating the bentportion with a laser for a predetermined time period.
 4. The method ofclaim 1, wherein the aluminum alloy is a 7XXX series aluminum alloy. 5.A roll-formed product of an aluminum alloy, the roll-formed productincluding the aluminum alloy and having a bent portion formed thereon,wherein the roll-formed product is formed by partially performing a heattreatment on the bent portion to remove residual stress which ispartially generated in the bent portion during the forming.
 6. Theroll-formed product of claim 5, wherein the heat treatment is performedby heating the bent portion at a temperature of 350 to 400° C.
 7. Theroll-formed product of claim 6, wherein the heat treatment is performedby irradiating the bent portion with a laser for a predetermined timeperiod.
 8. The roll-formed product of claim 5, wherein the aluminumalloy is a 7XXX series aluminum alloy.
 9. The roll-formed product ofclaim 5, wherein the product is configured such that in a microstructurehaving a ratio of an angle boundary with a tilt angle equal to orgreater than 2° and less than 15° is equal to or less than 21%.
 10. Theroll-formed product of claim 5, wherein the product is a bumper beam fora vehicle.